An important design consideration in enhanced oil recovery (EOR) processes is the determination of the conditions under which the dynamic miscibility may be reached for certain fluids and specific characteristics in a reserve. Temperature of the process is established by the reserve conditions but the pressure may be controlled within certain limits. Likewise, the injected gas composition may also be controlled. Therefore, the problem is only with the determination of the minimum miscibility pressure.
As its name indicates, the MMP is the minimum pressure at which the injection of a gas (CO2 or other gas) reaches its miscibility upon multiple contacts with crude oil for specific composition and temperature (Stalkup, 1984; Green and Willhite, 1998; Pedersen and Christensen, 2007).
Usually, the MMP for gas/oil mixtures has been measured by means of displacement experiments in a slim tube (Yellig and Metcalfe, 1980), the rising bubble apparatus (Christinasen, et al. 1986, 1987) and the technique of vanishing the interfacial tension (Rao, 1997). These experiments involve the oil contact by means of a gas at constant temperature (reservoir temperature, Tres) evaluating the dissolvent potential used for the oil recovery. The slim tube experiments are also made to determine the effects of gas injection speed, composition of both gas and oil in the displacement system and on recovery efficiently. Although the experiments to measure the MMP in the slim tube (Yellig and Metcalfe, 1980) are considered as standard in the oil industry, there is no standardized apparatus, process or criteria to determine the MMP with this technique. In addition to these disadvantages, the slim tube technique is extremely time consuming (4-5 weeks), requires the use of a lots of fluid (crude oil and injection gases), cleaning thereof among experiments with different oils is also extremely time consuming and solvents (xylene, toluene, dichloromethane, nitrogen, etc.) and, if organic matter precipitation occurs (asphaltenes), the precipitates may cover the packing pores and damage the slim tube, thus rendering it unusable.
U.S. Pat. No. 4,455,860 discloses an apparatus and method for determination of CO2 MMP of a reservoir oil. This apparatus is comprised of a capillary tube which is filled with crude oil at specific pressure (lower than the MMP and the reservoir temperature). The technique describes the injection of CO2 at a constant flow and the measurement of the pressure drop between the entrance and exit of the tube, where the experiment is repeated at different pressures. MMP value is inferred by the behavior during the pressure drop between the experimental values and the values calculated by the Hagen-Poiseuille equation under the same temperature and pressure conditions.
U.S. Pat. No. 4,627,273 relates to the apparatus and the rising bubble technique to measure the MMP of a gas in a liquid. The apparatus is basically comprised of a steel sight gauge which contains a cylindrical transparent tube resistant to pressure used to hold and to observe the flow of the fluids in question and a needle at the bottom thereof, this needle is used for injection of the displacement gas. The process for determination of the MMP consists of filling in the cylinder by the bottom part of the tube with distilled water at the intended pressure, then, crude oil is injected through the upper part of the tube so that the water is displaced, when the cylinder has been filled with oil, a gas bubble is formed in the bottom part of the tube. The buoyant force of the bubble causes its rising through the oil and the MMP is determined by the shape and color of the rising bubbles. The test is made at different pressures until finding the MMP, pressure under which the bubble vanishes due to the effect of the multiple contact with oils before reaching the upper part of the tube.
U.S. Pat. No. 4,621,522 relates to a method to determine the minimum enhancement an injection gas should have to reach the miscibility with the liquid hydrocarbon by observing the behavior of the bubbles rising by a cylindrical tube. It is partially related to a continuation-in-part of U.S. Pat. No. 4,627,273.
U.S. Pat. No. 5,505,074 relates to an apparatus and a method to determine the minimum miscibility pressure (MMP) of a gas in a liquid based on the pressure changes regarding the time in a rising bubble apparatus.
Another apparatus and technique used for determining the MMP is the technique of vanishing the interfacial tension (VIT) developed by Rao (1997). The apparatus to determine the MMP with the technique of vanishing the interfacial tension consists of a stainless steel sight gauge operating up to 70 MPa and 473 K. The sight gauge has two glass windows placed on the opposite sides enabling observation and collection of images profiles of the drop/bubble using a light source on one side and a camera system on the opposite side. The cameras are connected to a high resolution monitor and to a computer equipped with an image collection port and imaging analysis software. As well as with an axial drops symmetry analysis (ASDA) software interactively calculating the IFT. Equation used for the IFT calculation requires density values of the liquid and gas phases which are measured with a vibrating tube densimeter and they are calculated with a commercial simulator based on a compositional analysis of the gas phase, respectively.
U.S. Pat. No. 7,779,672 discloses a method and an apparatus to measure the minimum miscibility pressure between two phases for enhanced recovery processes or geologic storage of CO2. The method is based on the technique of vanishing the interfacial tension initially reported by Rao (1997) but making such technique faster and more independent from the measurement of densities of the liquid and gaseous phases.
Song et al. (2011) recently developed a technique to determine the minimum miscibility pressure of the n-decane+CO2 system. Based on a magnetic resonance imaging study, data obtained by this technique showed results comparable to the results of the technique developed by Rao (1997), to the results of Ayirala (2005) and to the results obtained by the rising bubble technique and the slim tube technique (Elsharkawy, 1996). One of the disadvantages of the magnetic resonance imaging is that the images obtained from the phases corresponding to the miscibility pressure are extrapolated to a zero value in the intensity of the signal and as in the technique of vanishing the interfacial tension, the MMP correspond to an extrapolation of a very good description of the behavior of a property regarding the pressure. In addition, no experiments for crude oil samples are reported.
Despite the above-mentioned techniques exist, most of the measurements of the MMP have certain disadvantages, among which the following may be found: extremely time consuming, high operation costs, use of great sample amounts, measurements are strongly subject to interpretation that the experimenter provides, (slim tube apparatus) since there are no standardized criteria or processes. Results are not based on any thermodynamic criteria (rising bubble apparatus) or the value reported from the MMP is an extrapolation (technique of vanishing the interfacial tension) of a good description of the property up to certain limit inherent to the apparatus. Therefore, an apparatus and a suitable technique are required for determination of the MMP under different temperature and pressure conditions for any type of hydrocarbon.
French patent application No. FR 1253235 discloses the application of the transitiometric technique for the oil industry. A process of fluid charge in a calorimetric cell is described in the patent. The patent discloses the application for determining transition from liquid to solid, liquid to steam and solid to liquid phases for crude oil of any concentration in a wide interval of temperature and pressure; the above to determine the surrounding precipitation of asphaltenes of any kind of oil. The French patent is a key background for this invention, which is hereby incorporated by reference in its entirety.